Antiretroviral therapy has extended the life expectancy of persons living with HIV (PLWH) and, as a result, age-related comorbidities are increasingly commonplace. This observation will impact the clinical outlook of HIV care and management in older adults. Multimorbidity, the occurrence of ?2 age-related comorbidities within the same individual, will invariably add to the already complex care needs of PLWH. However, there is limited research evaluating multimorbidity in the setting of HIV. Advancing current understanding is needed to formalize consideration of multimorbidity in HIV care guideline development, and respond to the National HIV/AIDS Strategy's call to improve long-term health outcomes in PLWH. Accordingly, this research aims to evaluate the epidemiology of multimorbidity and its influence on clinical care patterns among HIV-infected adults in the United States (US) and Canada from 2000-2010, with a particular interest in those with a history of injection drug use (IDU). Multimorbidity will be comprised of the following age-related comorbidities: hypertension, anemia, dyslipidemia, diabetes, renal impairment, myocardial infarction (MI), end-stage renal disease (ESRD), end-stage liver disease (ESLD), and non-AIDS malignancies. The proposed research will be nested within the North American AIDS Cohort Collaboration on Research and Design (NA-ACCORD), the largest cohort collaboration of HIV-infected adults in the U.S. and Canada. This population is well suited to address study objectives given its size, geographic heterogeneity, and demographic similarity to the broader population of PLWH in the US. Aim 1 will quantify the prevalence, incidence, and time between successive comorbidities, comparing multimorbidity in HIV-infected to-uninfected individuals. This aim will employ causal inference, competing risks, and multivariate survival analysis methods. Aim 2 will identify individual and regional-level factors that are associated with multimorbidity among HIV-infected individuals. This will be accomplished by multivariate survival analysis to address the correlated nature of comorbidities within individuals. Aim 3 will evaluate how multimorbidity impacts retention in care, adjusting for time-varying confounders, through causal inference modeling. For each aim, analyses will be stratified by individuals with a history of IDU and/or positive hepatitis C antibody. This population is of particular interest due to the excess burden of comorbidities that complicates their HIV care and treatment. This subgroup may especially benefit from targeted approaches to prevent multimorbidity and delay end-stage organ damage.